1. Field of the Invention
This invention relates to computer systems and more particularly to power management of computer systems.
2. Description of the Related Art
An important market criteria for microprocessors is the rated frequency at which the microprocessor operates, typically denominated in hundreds of MHz or GHz. Rated frequency is used as a surrogate for performance and higher performance processors command higher prices. Thus, it is advantageous to market processors with the highest possible frequency rating.
However, there are limitations as to how high one may rate a processor in terms of frequency. As the number of transistors provided on a single integrated circuit has increased and the clock speed of integrated circuits has increased, power and related thermal considerations have become an important consideration in computer design. Power and thermal considerations can limit processor performance in certain environments, particularly in mobile applications. Computer systems measure the die temperature or case temperature using sensors on or close to the processor. In order to keep processor die temperatures within safe bounds to avoid potential damage to the processor, passive and active cooling have traditionally been employed to control temperature. Passive cooling has been accomplished by reducing a processors clocks speed, either by throttling processor clocks (reducing effective frequency by turning clocks off for a predetermined period) or by reducing actual frequency of the clocks. Reducing the actual and/or effective frequency of an integrated circuit causes a reduction in power consumption and thus reduces temperature. In addition to reducing clock frequency, it is known in the art to reduce voltage in conjunction with reduced clock speed to achieve additional cooling.
Thus, the rated processor speed can be limited by power and thermal considerations and more specifically is determined, at least in significant part, by operating frequency, voltage, and temperature. If the die temperature can be kept sufficiently low, the rated processor clock speed can be increased. In one prior art approach to maximizing frequency at which a processor can be marketed, the processor includes an on-die temperature sensor that automatically throttles (reduces) the CPU's clock by 50% if the processor's temperature crosses a factory set threshold. If the maximum die temperature is set at a sufficiently low level of, e.g., 70 degrees C., the processor is able to meet the timing budget for the silicon process in which it is manufactured at a higher frequency at a given voltage than if the die temperature was significantly higher. The timing budget is the amount of time a signal has to propagate through combinational logic from one set of storage elements (e.g., flip-flops) to another set of storage elements and meet the setup and hold times associated with the flip-flops. At a lower temperature, a device can more easily meet a timing budget because, e.g., the propagation delays are reduced. Thus, to ensure that timing budgets are met, in the prior art approach described above, when the maximum die temperature associated with the device is crossed, e.g., 70 degrees C., the processor clock is throttled back to, e.g., 50% of its rated speed, which reduces the power consumption and thus the temperature. Thus, a processor rated at 2 GHz would run at 1 GHz upon crossing of the maximum die temperature threshold. In that way the processor can be rated at a higher speed at or below 70 degrees C., although it may operate with significantly reduced capability above that temperature.
While that allows the processor to be rated highly, it also results in a system that operates at a significantly lower clock speed in certain environments. It would be desirable to ship more higher speed rated processors without having to drastically sacrifice performance when the critical temperature threshold is crossed.